1. Field of the Invention
The present invention relates to a mobile communication terminal system, and more specifically to a mobile communication terminal device such as that in portable telephone sets and mobile phones capable of preventing pieces of information such as serial numbers each for use in charging control of subscribers and so on from being incorrectly rewritten and to a method of preventing information from being incorrectly rewritten.
2. Description of the Prior Art
Referring to FIG. 1, a prior art mobile communication terminal device is illustrated which comprises a CPU 2, an EEPROM 3 as a ROM capable of electrical writing/erasing, a ROM 4, a RAM 5, a TRX 6 as a transmitter/receiver section, an antenna 7, an interface(I/O) 8, and an I/O connector 9. Herein, upon writing in a storage region RG in the EEPROM 3 a serial number (hereinafter, referred to as ESN) for use in charging control and so on, the CPU 2 receives and analyzes ESN data fed through the I/O connector 9 and the interface 8, and thereafter writes the same in the storage region RG.
Once the mobile communication terminal device 1 calls a signal, the ESN is sent to a base station as data together with a MIN (Mobile Identification Number) as a subscriber's number. In the base station, it is judged on the basis of the received data whether coincidence is attained or not between the combination of the ESN and the MIN and a previously registered corresponding combination, and if so a subscriber identified by that number is charged. In this case, if any information concerning other person's ESN and MIN could incorrectly be obtained and his ESN and MIN could be written in a one's own mobile communication terminal 1, that person would be charged. The MIN is needed to be rewritten after shipping, but the ESN, which is a number inherent to the device, is not needed to be rewritten after it is once written upon shipping. Further, the ESN has no limitation to its writing, and hence it is rewritable with ease once a method of its writing is known.
Such a prior art mobile communication terminal device therefore has a difficulty in that an ESN written in an EEPROM is rewritten with ease by investigating a storage address of the ESN in the EEPROM and rewriting that data.